1. Technical Field of the Invention
The present invention relates to a technology for generating a masker sound signal to prevent sound from being overheard.
2. Description of the Related Art
Various technologies for preventing sound from being overheard using a masking effect have been proposed. The masking effect is a phenomenon in which, when two kinds of sound signals are transmitted to the same space, it is difficult for a person present in the space to perceive the respective sound signals according to a relationship between acoustic properties (a frequency component, a time waveform, etc.) of the sound signals. In most of such technologies, a masker sound is emitted to disturb hearing the voice of a speaker in a region adjacent to another region in which the speaker is present via a wall or a partition. In this case, the masker sound may be a noise having a spectrum over a wide band. However, it is known that a sound exhibiting properties similar to those of a sound as an object to disturb hearing (hereinafter, referred to as a target sound) achieves a high masking effect.
Patent Reference 1 discloses a masking system configured to select a masker sound exhibiting the highest masking effect from a plurality of kinds of masker sounds every time and to discharge the selected masker sound. The masking system disclosed in this patent reference prevents voices from being overheard between two regions, e.g., two acoustic spaces 20A and 20B, adjacent to each other while a wall is disposed between the two regions. In this masking system, voices of people having different ages, languages, and genders are previously collected. A scramble sound signal obtained by sorting a frame arrangement of a sound signal of each of the collected voices is stored in a memory together with acoustic property information indicating a format or a power spectrum of each of the voices corresponding to the scramble sound signal. In this masking system, the voice of a speaker present in the acoustic space 20A is analyzed to obtain acoustic property information thereof, a scramble sound signal, stored in the memory, corresponding to acoustic property information the most approximate to the obtained acoustic property information is read, and this scramble sound signal is discharged into the acoustic space 20B as a masker sound. According to this technology, a masker sound exhibiting properties the most approximate to the voice of a speaker present in the acoustic space 20A is discharged into the acoustic space 20B, with the result that it is possible to achieve a high masking effect in the acoustic space 20B.
Patent Reference 2 discloses a technology for processing a sound waveform of the voice of a person to prevent the voice of the person from being heard. A masking system disclosed in this patent reference divides a sound signal of the voice recorded through a microphone installed in a room into a plurality of sections each having a time length corresponding to each syllable, generates a masker sound signal by reversing samples in each section and sorting the arrangement of the sections, and discharges the generated masker sound signal into a neighboring room. The sound quality of the masker sound generated by this method is similar to that of the voice of a speaker; however, it is difficult for a person present in the neighboring room to understand contents of the original voice from the masker sound. Consequently, it is possible to discharge this masker sound into the region into which a target sound leaks, thereby achieving a high masking effect in the region.    [Patent Reference 1] Japanese Patent Application Publication No. 2008-233672    [Patent Reference 2] Japanese Patent Application Publication No. 2008-233671
In the masking system disclosed in Patent Reference 1, it is necessary to continuously and repeatedly discharge one scramble sound selected from a plurality of kinds of scramble sound signals into the acoustic space 20B as a masker sound so as to secure a masking effect in the acoustic space 20B. However, if the same masker signal is continuously and repeatedly discharge for a long period of time as described above, a person present in the acoustic space 20B may perceive that the same masker signal is repeatedly discharged, with the result that the person may feel a sense of incongruity.
In the masking system disclosed in Patent Reference 2, a sound waveform obtained by reversing a sample array of a sound waveform of the voice of a person exhibits an unnatural accent different from that of the original sound waveform. For this reason, if the sound waveform obtained by reversing the sample array of the sound waveform of the voice of the person is continuously discharged as a masker sound, a person present in a region into which the masker sound is discharged may feel a sense of incongruity.